In recent years, in the field of servers and high-performance computers, an improvement in performance causes increased transmission capacity of I/O functions that allow a CPU and an external interface to communicate with each other. On the other hand, an optical interconnect technology that enables high-capacity transmission with optical signals by arrangement of a photoelectric conversion element has been considered in the field of a conventional high-speed transmission using electricity from the viewpoint of cross talk or wiring density.
In the optical interconnect technology, a small-sized optical module is smaller and more cost effective in production than that of a conventional infrastructural optical communication. For example, this kind of optical module, which has been known in the art, is one in which an optical element, such as a light-emitting element or a light-receiving element, is mounted face-down on a circuit board containing a transparent material and an optical waveguide is arranged on the surface thereof opposite the surface on which the optical element is mounted.
This kind of optical module is designed to be manufactured at lower cost. Thus, an optical module, where an optical transmission element is mounted on a transparent FPC substrate comprised of a thin film made of polyimide or the like, has been known. For example, the above optical module is placed on a printed circuit board in a server through an electric connector, so that a high-speed transmittable optical communication may be realized at low cost.
In the field of an optical interconnect technology, high-speed optical transmission of more than 20 GBps has been increasingly desired. Therefore, a light-emitting element which is operable at high speed, and an optical module on which a light-receiving element is mounted have been used. In the aforementioned optical module, however, a large coupling loss of optical signals has been observed between the light-emitting element or a light-receiving element and the optical waveguide.
Specifically, the aperture of the light-emitting part or light-receiving part of the optical element is not always equal to an input aperture or output aperture of the optical waveguide. The larger the difference between the apertures, the more signal light beams are emitted outside. Thus, coupling loss increases. In particular, the aperture of the light receiving surface (light receiving surface area) becomes small as the light-receiving element is operated at higher speed. To drive the light reception element at higher speed, the stray capacity of the light-receiving element is preferably set lower. Thus, it is preferable to make the aperture of the light receiving surface smaller. For example, it is preferable to make the aperture of the light receiving surface less than 50 μm if the light-receiving element is desired to be operated at 20 GBps or more.
On the other hand, an optical waveguide has an outlet aperture of about 50 μm in general when a cost-effective multimode waveguide is used. Therefore, considering the spread of emitting beams from the multimode waveguide, the optical waveguide may cause an optical loss of about 2 to 3 dB. Even in the case where an optical element is a light-emitting element, it is preferable to thicken a substrate in order to respond to high frequencies. Some emitting beams have difficulty coupling with the optical waveguide when the substrate is thickened, leading to an optical decrease of about 0.5 to 1 dB.
To prevent this kind of optical loss, a configuration of an optical element in which a microlens is formed on the light receiving surface of a light-receiving element by using a dispenser to integrally form a light-receiving element and a lens are integrally formed (see, for example, Japanese Laid-open Patent Publication No. 2004-241630); a method for integrally forming a concave mirror on a 45-degree mirror portion of a waveguide (for example, Japanese Laid-open Patent Publication No. 2001-141965); and a method for inserting a lens into an opening drilled in a printed circuit board (see, for example, Japanese Laid-open Patent Publication No. 2006-284781).